Handle next track not found consistently

Highlight in red, don't set as next track.
Fixes #51

app/musicmuster.py

@@ -415,7 +415,10 @@ class Window(QMainWindow, Ui_MainWindow):
             self.music.play(self.current_track.path)
 
             # Update metadata
-            # TODO is this valid if next track is on different playlist?
+            # Get next track for this playlist. May be None if there is
+            # no automatic next track, and may later be overriden by
+            # user selecting a different track on this or another
+            # playlist.
             next_track_id = self.current_track_playlist_tab.play_started()
 
             if next_track_id is not None:
@@ -423,7 +426,7 @@ class Window(QMainWindow, Ui_MainWindow):
                 self.next_track_playlist_tab = self.current_track_playlist_tab
                 # Check we can read it
                 if not self.file_is_readable(self.next_track.path):
-                    self.show_warning(
+                    helpers.show_warning(
                         "Can't read next track",
                         self.next_track.path)
             else:
@@ -501,11 +504,6 @@ class Window(QMainWindow, Ui_MainWindow):
 
         self.visible_playlist_tab().select_unplayed_tracks()
 
-    def show_warning(self, title, msg):
-        "Display a warning to user"
-
-        QMessageBox.warning(None, title, msg, buttons=QMessageBox.Cancel)
-
     def song_info_search(self):
         """
         Open browser tabs for Wikipedia, searching for

app/playlists.py

@@ -357,8 +357,10 @@ class PlaylistTab(QTableWidget):
         # Scroll to put current track in centre
         scroll_to = self.item(current_row, self.COL_INDEX)
         self.scrollToItem(scroll_to, QAbstractItemView.PositionAtCenter)
-        next_track_id = self._mark_next_track()
-        self._repaint()
+
+        # Get next track
+        next_track_row = self._find_next_track_row()
+        next_track_id = self._set_next(next_track_row)
         return next_track_id
 
     def play_stopped(self):
@@ -648,13 +650,13 @@ class PlaylistTab(QTableWidget):
             and pos.y() >= rect.center().y()        # noqa W503
         )
 
-    def _mark_next_track(self):
+    def _find_next_track_row(self):
         """
         Find next track to play.
 
         If not found, return None.
 
-        If found, mark row with metadata and return track_id.
+        If found, return row number.
         """
 
         found_next_track = False
@@ -667,16 +669,14 @@ class PlaylistTab(QTableWidget):
         for row in range(start, self.rowCount()):
             if row in notes_rows:
                 continue
-            self._meta_set_next(row)
             found_next_track = True
             break
 
-        if not found_next_track:
+        if found_next_track:
+            return row
+        else:
             return None
 
-        track_id = self._get_row_id(row)
-        return track_id
-
     def _meta_clear(self, row):
         "Clear metadata for row"
 
@@ -813,11 +813,13 @@ class PlaylistTab(QTableWidget):
         track_id = self._get_row_id(row)
         if track_id:
             if self._track_path_is_readable(track_id):
-                self._meta_set_next(self.currentRow())
+                self._meta_set_next(row)
                 self.master_process.set_next_track(track_id)
             else:
-                self._meta_set_unreadable(self.currentRow())
+                self._meta_set_unreadable(row)
+                track_id = None
             self._repaint()
+        return track_id
 
     def _repaint(self, clear_selection=True):
         "Set row colours, fonts, etc"